FIG. 1 (Prior Art) is a simplified circuit diagram of a flyback converter power supply 1. Flyback converter 1 generates a 5.0 volt DC voltage from a 110 volt AC source. 110 volts AC supplied from source 2 is present between connectors 3 and 4. The 110 volt AC voltage is rectified by a full wave bridge rectifier comprising diodes 5-8. Capacitor 9 is a smoothing capacitor. A rough DC voltage VIN of approximately the 150 volt peak voltage of the 110 volt AC RMS input signal is present on conductor and node 10. A switch 11 is rapidly switched on and off to pull pulses of current through the primary 12 of a transformer 13 from this VIN conductor. When a pulse of current is pulled through the primary 12, an amount of energy is stored in the transformer. When the switch 11 is then opened, a pulse of current is made to flow from the secondary 14 so that energy stored in the transformer is transferred to the load 15. The current from the secondary 14 flows through a rectifier diode 16. Such pulses of current keep charge on a storage capacitor 17 so that the desired 5.0 volts DC is maintained across load 15 between VOUT conductor 18 and ground conductor 19. Standard sensing and control circuitry that controls the switching of switch 11 is not illustrated in order to simply the diagram. The flyback topology of FIG. 1, including its sensing and control circuitry, is well known in the art.
FIG. 2 (Prior Art) is a set of simplified waveform diagrams. These diagrams set forth waveforms of voltages and currents present in the circuit of FIG. 1. The upper waveform labeled VS shows the voltage present across switch 11. From time t0 to time t1, switch 11 is closed. Current is flowing from node 10, through the primary 12, and through switch 11, and to ground node and conductor 20. From time t0 to time t1 this current increases as illustrated in the waveform labeled IS. From time t0 to time t1, energy is being stored in the transformer. Switch 11 is closed. Accordingly, the voltage across switch 11 is zero. Magnetic flux is building in the transformer as indicated by the waveform labeled “MAGNETIC FLUX”. Then at time t1, switch 11 is opened. The opening of switch 11 causes a current to stop flowing in the primary, and to start flowing in the secondary. As illustrated in the fourth waveform labeled ID, this current flowing in the secondary decreases over time. The magnetic flux in the transformer decreases as well. At time t2, there is no more energy stored in the transformer and the secondary current stops flowing. From time t2 to time t3, there is little or no current flow in either the primary or the secondary of the transformer as indicated by the IS and ID waveforms. The switching cycle repeats at time t0 when switch 11 is closed again to start the next cycle. The switching period from time t0 of one period to time t0 of the next period may, for example, be ten microseconds.
FIG. 3 (Prior Art) illustrates current flow from time t2 to time t0. Reference numeral 21 identifies the split core of transformer 13. FIG. 4 (Prior Art) illustrates current flow from time t0 to time t1. FIG. 5 (Prior Art) illustrates current flow from time t2 to time t2.
When current is flowing from the secondary winding of the transformer and to capacitor 17 and load 15, the current is flowing through rectifier diode 16. The rectifier diode 16 being in the current path results unwanted power dissipation. At a given time, the instantaneous power dissipated in rectifier diode 16 is the product of the instantaneous current flow through the diode and the instantaneous voltage being dropped across the diode. Average power dissipation in rectifier diode 16 is the average of such instantaneous power dissipation taken over the entire switching cycle of the flyback converter. In a common conventional flyback converter that outputs 20 amperes at 5.0 volts DC such as the flyback converter illustrated in FIG. 1, the forward voltage drop VF of the rectifying diode at its rated current flow is approximately 1.0 volts. Average power dissipation in the rectifying diode may be approximately 15 Watts.